Electrochemical cells for lithium ion batteries are as standard constructed in the discharged condition. The advantage of this is that both electrodes are present in an air- and water-stable form. The electrochemically active lithium is here exclusively introduced in the form of the cathode material. The cathode material contains lithium metal oxides such as for example lithium cobalt oxide (LiCoO2) as an electrochemically active component. The anode material in the currently commercial batteries contains, in the discharged condition, a graphitic material having a theoretically electrochemical capacity of 372 Ah/kg as the active mass. As a rule, it is completely free of lithium. In future designs, also materials (also free of lithium) having a higher specific capacity may be used, for example alloy anodes, frequently on the basis of silicon or tin.
In real battery systems, part of the lithium introduced with the cathode material is lost as a result of irreversible processes, above all during the first charging/discharging process. Moreover, the classical lithium ion battery design with lithium-free graphite as the anode has the disadvantage that lithium-free potential cathode materials (e.g. MnO2) cannot be used.
In the case of graphite it is assumed that above all oxygen-containing surface groups react, during the first battery charging process, irreversibly with lithium to form stable salts. This part of the lithium is lost for the subsequent electrochemical charging/discharging processes, because the salts formed are electrochemically inactive. The same applies to the case of alloy anodes, for example silicon or tin anode materials. Oxidic impurities consume lithium according to:MO2+4Li→M+2Li2O  (1)
(M=Sn, Si and others)
The lithium bound in the form of Li2O is no longer electrochemically active. If anode materials having a potential of < approx. 1.5 V are used, a further part of the lithium is irreversibly consumed on the negative electrode for the formation of a passivation layer (so-called solid electrolyte interface, SEI). In the case of graphite, a total of approx. 7 to 20% by weight of the lithium introduced with the positive mass (i.e. the cathode material) is lost in this way. In the case of tin and silicon anodes, these losses are usually even higher. The “remaining” transition metal oxide (for example CoO2) delithiated according to the following equation (2) cannot, due to a lack of active lithium, make any contribution to the reversible electrochemical capacity of the galvanic cell:2nLiCoO2+MOn→nLi2O+M+2nCoO2  (2)
(M=Si, Sn etc.; n=1 or 2)
There have been many examinations with a view to minimise or completely compensate these irreversible losses of the first charging/discharging cycle. This limitation can be overcome by introducing additional lithium in a metallic form, for example as a stabilised metal powder (“SLMP”) into the battery cell (e.g. US2008283155A1; B. Meyer, F. Cassel, M. Yakovleva, Y. Gao, G. Au, Proc. Power Sourc. Conf. 2008, 43rd, 105-108). However, the disadvantage of this is that the usual methods for producing battery electrodes for lithium ion batteries cannot be carried out. Thus, according to the prior art, passivated lithium reacts with the main air components of oxygen and nitrogen. Although the kinetics of this reaction are very decelerated compared to non-stabilised lithium, however, after prolonged exposure to air, also under dry room conditions, a change in the surfaces and a decrease in metal content cannot be avoided. The extremely vehement reaction of Li metal powder with the solvent N-methyl-pyrrolidone (NMP), which is often used for preparing electrodes, has to be regarded as an even more serious disadvantage. Although significant progress in the direction of a safer handling could be made by providing stabilised or coated lithium powders, however, the stability of the lithium powder stabilised according to the prior art was frequently not sufficient in order to guarantee, under practical conditions, a safe use of passivated lithium powder in the case of NMP-based electrode production methods (suspension methods). Whilst uncoated or deficiently coated metal powders may vehemently react with NMP even at room temperature as early as after a brief induction period (thermal run away), in the case of coated lithium powder this process will occur only at elevated temperatures (for example 30 to 80° C.). Thus, US2008/0283155 describes that the lithium powder coated with phosphoric acid from example 1 reacts extremely vehemently (run away) immediately after mixing them together at 30° C., whereas a powder additionally coated with a wax at 30° C. in NMP will be stable for at least 24 h. The lithium powders coated according to WO2012/052265 are kinetically stable in NMP up to approx. 80° C., however, they decompose exothermically at temperatures beyond that, mostly under phenomena of the run away type. For mainly this reason, the use of lithium powders as a lithium reservoir for lithium ion batteries or for pre-lithiation of electrode materials has so far been commercially unsuccessful.
Alternatively, additional electrochemically active lithium can be introduced into an electrochemical lithium cell also by adding graphite lithium intercalation compounds (LiCx) to the anode. Such Li intercalation compounds may be produced either electrochemically or chemically.
The electrochemical production is carried out automatically during charging of conventional lithium ion batteries. As a result of this process, materials with a lithium:carbon stoichiometry of no more than 1:6.0 may be obtained (see e.g. N. Imanishi, “Development of the Carbon Anode in Lithium Ion Batteries”, in: M. Wakihara and O. Yamamoto (ed). in: Lithium Ion Batteries, Wiley-VCH, Weinheim 1998). The partially or fully lithiated material produced in this way can in principle be taken from a charged lithium ion cell under a protective gas atmosphere (argon) and can be used, after appropriate conditioning (washing with suitable solvents and drying), for new battery cells. Due to the extensive efforts associated with this, this approach is chosen only for analytical examination purposes. For economic reasons, this method has no practical relevance.
Further, there are preparative chemical approaches for lithiating graphite materials. It is known that lithium vapour reacts with graphite at a temperature starting from 400° C. to form lithium intercalation compounds (lithium intercalates). However, once 450° C. is exceeded, undesired lithium carbide Li2C2 forms. The intercalation reaction works well with highly oriented graphite (HOPG=Highly Oriented Pyrolytic Graphite). If liquid lithium is used, a temperature of just 350° C. is sufficient (R. Yazami, J. Power Sources 43-44 (1993) 39-46). The use of high temperatures is generally unfavourable for energetic reasons. Added to this, in the case of the use of lithium, are the high reactivity and corrosiveness of the alkali metal. Therefore, this production variant is also without any commercial significance.
In the case of the use of extremely high pressures (2 GPa, corresponds to 20,000 atm), lithium intercalation can be achieved even at room temperature (D. Guerard, A. Herold, C. R. Acad. Sci. Ser. C., 275 (1972) 571). Such high pressures can be achieved only in highly specialised hydraulic presses which are suitable only for the production of minute laboratory-scale quantities. This means that this is not an industrially suitable method for producing commercial quantities of lithium graphite intercalation compounds.
Finally, the production of lithiated natural graphite (Ceylon graphite) by means of high energy grinding in a ball mill has been described. To this end, the predominantly hexagonally structured natural graphite from today's Sri Lanka is reacted with lithium powder (170 μm average particle size) in Li:C ratios of 1:6; 1:4 and 1:2. A complete lithiation into the final molar ratio LiC6 could be achieved only with a molar ratio of 1:2 (R. Janot, D. Guerard, Progr. Mat. Sci. 50 (2005) 1-92). This synthesis variant is also disadvantageous from a technical and commercial point of view. On the one hand, a very high lithium excess is needed in order to achieve a sufficient or complete lithiation. The vast majority of the lithium is lost (in the mill or on the grinding balls) or is not intercalated (i.e. is still present in the elementary form). On the other hand, as a rule no unconditioned natural graphite is used for the production of anodes for lithium ion batteries. The reason is that the mechanical integrity of natural graphite is irreversibly destroyed during battery cycles as a result of so-called exfoliation by the intercalation of solvatised lithium ions (see P. Kurzweil, K. Brandt, “Secondary Batteries—Lithium Rechargeable Systems” in Encyclopaedia of Electrochemical Power Sources, J. Garche (ed.), Elsevier Amsterdam 2009, vol. 5, pages 1-26). Therefore, more stable synthetic graphites are used. Such synthetic graphites are less crystalline and have a lower degree of graphitisation. Finally, the long grinding times of preferably 12 hours (page 29) that are needed for natural graphites are of disadvantage.
For the reasons mentioned above, the method described has never been commercialised.
In the publication by Janot and Guerard as listed above, also the application properties of the lithiated Ceylon graphite are described (chapter 7). Electrode production is carried out by simply pressing the graphite onto a copper network. As a counter and reference electrode, lithium strips are used, as the electrolyte, a 1 M LiClO4 solution in EC/DMC is used. The type of electrode preparation by simple pressing on does not correspond to the prior art as applied in commercial battery electrode production. A simple compression without the use of a binder and, if necessary, adding conductivity additives, does not result in stable electrodes since the volume changes occurring during charging/discharging will by necessity lead to crumbling of the electrodes, as a result of which the functionality of the battery cell is destroyed.
A further disadvantage in connection with the use of graphite lithium intercalates consists in the fact that the latter behave reactively towards functionalised solvents, for example ethylene carbonate (EC), i.e.
they can react exothermically. Thus, the mixture of Li0.99C6 and EC shows a clearly exothermic behaviour when exceeding 150° C. Similar mixtures with diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate react as early as from approx. 110° C. with lithium graphite intercalation compounds (T. Nakajima, J. Power Sources 243 (2013) 581).